1. Field of the Invention
The present invention relates to a power convertor, and more particularly, to a resonant power converting circuit.
2. Description of Related Art
Because of the environmental protection awareness and the global warming issues, power consumption has been become one of the heavily discussed topics in many places. U.S. Environmental Protection Agency, EPA, defines energy efficiency regulations for a variety of electronic devices. For example, the efficiency level certifications of PC power supply are classified to a basic 80 PLUS level (80%, 80%, 80%), an 80 PLUS bronze level (82%, 85%, 82%), an 80 PLUS silver level (85%, 88%, 85%), and an 80 PLUS gold level (87%, 90%, 87%). As such, the topic of enhancing power converting efficiency has been become a major task that the suppliers of the power supply units is facing. Moreover, it is widely perceived that the power converting efficiency for the current pulse width modulation (PWM) controller, such as a forward PWM controller or other configurations, is not able to meet the energy efficiency regulations. In general, a front end DC/DC power convertor includes a PWM power convertor and a resonant power convertor. Since switch of the PWM power convertor belongs to hard-switch convertor topologies, the switching loss would be easily generated and would not improve the power converting efficiency. Therefore, resonant power convertors were developed to overcome the aforementioned undesirable problem of switching loss, since the resonant power convertor is associated with a soft-switching mechanism.
Conventional resonant power convertors could be primarily divided into three categories: (1) a series resonant convertor (referred to as “SRC”), (2) a parallel resonant convertor (referred to as “PRC”), and (3) a series-parallel resonant convertor (referred to as “SPRC” or called “LLC”). A controller of the resonant power convertor outputs a switching signal to a power switch of the resonant power convertor for controlling an output voltage of the resonant power convertor. However, since the conventional controller of the resonant power convertor has limits of switching frequency, the convertor gain would be limited and easily have issue of insufficient output voltage